ETL 1110-1-189
14 Feb 03
Summary:
Design Subgrade CBR = 1
(Based on measured values.)
Applicability: Both Geotextile & Geogrid
(Section 2.1 and Table 5)
1 CBR Subgrade = 4.8 psi Shear Strength, C (Figure 4)
Traffic = 2,000 passes 37.5-kip tandem-axle gear
(Use Figure 7)
Unreinforced Nc = 2.8 Reinforced Nc = 5.8 (Table 5)
Initial: tunreinforced = 24 in. treinforced = 14 in. (Figure 7)
Traffic Adjustment: 2,000 passes = +10% thickness
(Section 2.2.2)
Design: tunreinforced = 26 in. treinforced = 15 in.
Example A.1.2:
Description: Determine the reinforced design of an unpaved road for an area located in a silt
plateau in Afghanistan. Estimates of the potential traffic include approximately 1,000 passes of
heavily loaded M54A2C 5-ton cargo trucks. An expedient site investigation produced the
following in situ CBRs from DCP measurements for the upper 18-in. of the subgrade: 3, 4, 6,
and 5 CBR.
Solution: The design subgrade CBR is 3 since 75 percent of all measured values exceed the
3 CBR measurement. A geosynthetic applicability assessment based upon the design subgrade
CBR indicates that a geotextile is required for separation since the subgrade is fine-grained with
a CBR less than 4.0. A geogrid is may be cost-effective and should be considered. Entering
Figure 4 with a 3 CBR and drawing a horizontal line to the intersection of the shear strength
scale produces a design subgrade shear strength, C, of 10.1 psi. The design vehicle is identified
as 1,000 passes of an M54A2C 5-ton cargo truck. Table 4 indicates that the typical tandem-axle
gear weight is approximately 16 kips (Use the 17,500-lb curve in Figure 7). The unreinforced
bearing capacity factor, Nc, is 2.8 according to the text and Table 5. The reinforced bearing
capacity factor (Nc) for use with both a geotextile and a geogrid is 5.8 according to Table 5. The
effective subgrade bearing capacity, CNc, of the unreinforced subgrade is 28.3 psi, and the
reinforced subgrade is 58.6 psi. Using Figure 7, the required aggregate thicknesses for the
unreinforced and reinforced designs are 8 and 4 in., respectively. However, a minimum design
aggregate thickness of 6 in. is recommended according to Section 2.2.4. Thus, the final design
unreinforced and reinforced aggregate thicknesses are 8 and 6 in., respectively. The net
reduction in aggregate thickness requirements based upon the inclusion of the geotextile
separator and geogrid reinforcement is 2 in. of aggregate, a 25 percent reduction in required
aggregate thickness. Although 2 in. of aggregate savings seems trivial, the actual volume of
savings may be significant if the road length is extensive. A life-cycle cost analysis should be
performed to ensure a cost-effective design. In this case, a design using a geotextile without the
geogrid should also be conducted as an alternative design. As noted in Table 5, the Nc for a
geotextile alone is 5.0 based upon TM 5-818-8. Recent research has indicated that a
conservative geotextile reinforcement bearing capacity factor of 3.6 may be more appropriate.
For this example, the existing TM 5-818-8 criteria is used for a less conservative answer. Thus,
the bearing capacity (CNc) of the subgrade can be computed as 50.5 psi. Using Figure 7, the
required aggregate thickness over the geotextile is 4.5 in. Based on the minimum aggregate
thickness recommended in Section 2.2.4, the final design thickness for the geotextile only
alternative would be 6 in. Therefore, in this design example, the alternative design incorporating
only the geotextile would be recommended. Sample specifications for the geotextile and
geogrid are provided in Tables 2 and 3, respectively.
A-2
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